1. Field of the Invention
The present invention relates to a magnet assembly, and more particularly, to a magnet assembly for a high temperature application.
2. Statement of the Problem
It is known to use Coriolis mass flow meters to measure mass flow, density, and volume flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flow meters have one or more flow tubes of different configurations. Each conduit configuration may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial and coupled modes. In a typical Coriolis mass flow measurement application, a conduit configuration is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit.
The vibrational modes of the material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter. The material is then directed through the flow tube or flow tubes and exits the flow meter to a pipeline connected on the outlet side.
A driver applies a force to the flow tube. The force causes the flow tube to oscillate. When there is no material flowing through the flow meter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the different points. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
A flowtube driver typically comprises a coil that is opposed by a fixed magnet. The coil and the fixed magnet are attached to a pair of flowtubes or a flowtube and a balance beam. In operation, the magnetic field in the driver coil is rapidly alternated. The fixed, opposing magnet assists in generating oscillating forces that alternatively brings the flowtube(s) together and apart.
Likewise, a pickoff sensor of a flow meter can comprise a magnetic coil pickup and an opposing magnet, with one or both being attached to flowtubes, as described above. In operation, the magnetic coil pickup generates a substantially sinusoidal signal from the moving magnet when the flowtube(s) is oscillating.
A flow meter can be used to measure a flow material in a high temperature application. Some flow meter applications require continuous use in temperatures at or above 400 degrees Fahrenheit. In the prior art, magnets used for the driver and/or pickoff sensor assemblies are held in a magnet keeper by means of a shrink fit. The magnet keeper is attached to a flow tube.
In the prior art, Aluminum-Nickel-Cobalt (AlNiCo) magnets are used in high temperature applications where flow tube stiffness is low and resulting vibrational amplitudes are high. A drawback in using AlNiCo magnets is that AlNiCo magnets have relatively higher masses than other magnet technologies while their B-field strength is relatively low. However, in newer flow meter designs, stiffness is high and vibrational amplitudes are low. As a consequence, newer flow meter designs require low mass driver and pickoff systems in order to operate properly. Simply resorting to larger magnets is not practical because the spacing between brackets is fixed by the design of the flow meter.